# [Gmsh] 3 little questions

thierry.gilles at tiscali.nl thierry.gilles at tiscali.nl
Fri May 6 10:52:59 CEST 2005

```Hello

I've got 3 little question, sir..

I wrote you one year ago about a "normal" problem, and you gave me a satisfying
solution at that time (I used version 1.46 then).. But the problem is still
present, even with your (1.46?) solution, now I use the 1.60 version.. :(

Shortly :
I want to master the sense of normals of the elements of a mesh (e.g : all
directed outwards for a simple closed surface). Obviously, the sense of normals
depicted in geometry mode doesn't follow the corkscrew rule, so you cannot
trust this representation to invert signs in a "physical surface". Even worse,
the sense of the normals of a mesh as depicted after meshing cannot be trusted
either. It is only the sense of normals as depicted when reopening the SAVED
.msh file that the normals appear with respect to the corkscrew rule. It
is only looking at this saved file that it becomes possible to group all
elementaries in a physical surface WITH THE PROPER SIGN to get all normals
in the desired sense.. I didn't find that solution optimal at the time, but
it worked as you said with a cube or a sphere.. (I know I should participate
and develop a fix myself, but I'm really not that skilled ;( to start dealing
with your great code..sorry sorry for that ! )

Today I created a (slightly) complexer shape, using extrusion, symmetry and
rotation : XPopoComplete.geo.
There is no way I can get all normals to the exterior this time, even following
the scheme above !!!

Question 1 : Can you help me get all normals outwards for this "XPopoComplete"
example ?

Actually, I noted that the use of Rotation respects the orientation of the
surfaces (Duplicata sense = original sense), but the surfaces orientations
are inverted with Symmetry.. Quite logical actually, but..

Question 2 :
Is there a way to force the transformations to respect the original surfaces
orientation, or to globally invert it ?
I tried with Physical surface, as you can see, but it won't work this time
!!

And a last question : to group in a physical surface surfaces created with
a transformation is quite a job, as the surface numbers have to be mentioned,
but they are automatically chosen by the transformation.. The only way out
I found so far is to represent separately the Duplicata (filtering in "Visibility"),
read the surface numbers on screen and manually copy them in the .geo file..
Goes well for my simple example, but if the Duplicated surface contains
tens of or even more surfaces, this becomes a heavy task.

Question 3 : Is there a better way, eg forcing the numbering of transformed
entities/surfaces ?

Thierry GILLES
CDT
Thierry GILLES

_____________________________________________________________________

Je eigen persoonlijk e-mailadres bij Tiscali? Registreer nu
een domeinnaam bij Tiscali. Ga naar http://www.tiscali.nl

```